1. Field of the Invention
The present invention relates to a fixing unit used for an electrophotographic apparatus or the like.
2. Description of the Related Art
As one type of fixing unit for use in a recording apparatus using a known electrophotographic process, there is known a so-called heat fixing system, which includes a fixing roller and a pressure roller forming a nipper in cooperation with the fixing roller, in which a recording material carrying an unfixed image is conveyed while being nipped by the nipper, and the image is then fixed onto the recording material.
As means for realizing high-speed fixing by using the fixing unit of the heat fixing system, there are known, for example, the following systems. One system is that a recording material is conveyed wound around a fixing roller, and the recording material is heated in a region where no nipper exists, as disclosed in Japanese Utility Model Examined Publication No. Sho 53-39235. Another system is that a plurality of pressure rollers are provided so that a recording material is heated by a plurality of nippers, as disclosed in Japanese Patent Examined Publication No. Hei 5-63795.
As described above, a variety of high-speed fixing units have been proposed. The structure of the foregoing fixing units may suffer from the problem that the recording material is out of close contact with the fixing roller in a region between two pressure rollers, causing a slack in the recording material. The slack results from the extension of the recording material due to heating by the fixing roller, or results from a difference in speed between the fixing roller and the recording material at the outlet of the nipper where the recording material is nipped.
At the stage where the recording material has passed a first pressure roller where the recording material is initially nipped, the image is insufficiently fixed onto the recording material. Therefore, toners on the recording material are transferred (offset) onto the fixing roller. If slack occurs in the recording material, when the recording material passes a second pressure roller where the recording material is subsequently nipped, the toners transferred (offset) onto the fixing roller return to the recording material. However, because a distance of travel of the original image on the slack recording material is longer than that of travel of the offset toner on a surface of the fixing roller, the offset toner is retransferred onto the recording material before the original image passes the nipping portion of the second pressure roller. That is, the position of the recording material onto which the offset toner is retransferred is forward of the position of the original image on the recording material in a conveyance direction of the recording material. This blurs the image on the recording material. Once a slack occurs in the recording material, because the recording material is nipped between two pressure rollers, the slack is not eliminated until the operation of fixing the recording material by the first or second pressure roller is suspended or terminated.
In the foregoing conventional fixing units, attention is insufficiently given to the flapping or slack of the recording material, which is caused when the recording material is conveyed, resulting in problems such that an image is blurred or image quality is low in strength of fixation.